Field of the Invention
The present invention relates to a separator for use in a unit cell of a fuel cell having an electrolyte electrode assembly, and further relates to a method for producing the separator.
Description of the Related Art
In general, a fuel cell has a unit cell containing a pair of separators and an electrolyte electrode assembly sandwiched therebetween. In the unit cell, sealants, which contain a stack of a primer and a rubber, are disposed on the edges of both surfaces of the separators. Thus, the sealants are interposed between the separators or between the separator and the electrolyte electrode assembly.
In operation of the fuel cell, a fuel gas containing hydrogen is supplied to an anode, and an oxygen-containing gas is supplied to a cathode in the electrolyte electrode assembly. The sealants act to prevent leakage of the fuel gas and the oxygen-containing gas to the outside of the fuel cell.
For example, the separator is produced by press-forming a metal material such as a stainless steel. A silicone rubber has been widely used as the rubber in the sealant. The silicone rubber is highly elastic and easily compatible with swelling and shrinkage of the stack at the start and stop of the fuel cell. Furthermore, the silicone rubber maintains the elasticity even at a sub-zero temperature, and can act to prevent the leakage of the reactant gas even in a cold region. Thus, the silicone rubber is suitable for use in a vehicle fuel cell.
As described above, the primer is interposed between the rubber and the separator. The primer acts to achieve a satisfactory adhesion strength between the rubber and the separator.
However, the silicone rubber is not sufficient in acid resistance. In general, an electrolyte membrane in the electrolyte electrode assembly has a high acidity. Therefore, it is pointed out that the sealant (the silicone rubber therein) may be deteriorated and the elasticity may be lowered in the vicinity of the electrolyte membrane. Also the adhesion between the primer and the metal may be deteriorated by acid, and thus the sealant may be peeled off from the separator.
In view of solving this problem, the applicant has proposed a Cr-rich layer formed on the surface of the separator in Japanese Patent No. 4485552. In the case of using the Cr-rich layer, the primer is strongly bonded to the separator, so that the sealant is hardly peeled from the separator.